1. Field of the Invention
The present invention relates to electrochemical systems employing bipolar electrodes, and more particularly to improvements in various components of such a system, including such components as the electrodes, electrochemically active material contained on the electrodes, ionically conductive separators between electrodes, and seals used in containing the liquid electrolytes in the system, to produce a system capable of generating an exceptionally high output current density over short periods of time.
2. State of the Art
Electrochemical systems employing bipolar electrodes are well known, having been used by Faraday as early as 1839. Such a system comprises a stack or pile of electrodes comprising electronically conductive layers or plates with appropriate electrochemically active material applied to the opposite, broad surfaces or sides of the conductive layers or plates. Alternating with the electrodes in the stack or pile are ionically conductive layers or zones which contain an appropriate electrolyte. Means must be provided, of course, for retaining the electrolyte in the layers or zones between the electrodes, with the electrolyte making electrical contact with the respective electrodes.
It is known to form the conductive layer or plate of the electrode from electrically conductive plastic. See U.S. Pat. No. 4,098,967 wherein a conductive substrate is disclosed which is made of fluorocarbon polymer. The polymer is filled with finely divided vitreous carbon. Production of the filled polymer is accomplished by mixing the polymer in finely divided particles with the particles of carbon. The mixture is then set by compression molding at elevated temperature and pressure.
Absorbent separators for placement between electrodes are disclosed in U.S. Pat. No. 3,862,861. The absorbent separators disclosed in that patent are made from microporous rubber, polyvinyl chloride, polyolephins, phenolic resin impregnated paper and fiber glass material, with the latter being taught as the preferred material.
U.S. Pat. No. 3,862,861 further discloses pasting of various electrochemically active materials to the substrate of the electrodes. U.S. Pat. No. 4,098,967 teaches that improved bonding of the electrochemically active material is greatly improved by bonding a layer of lead alloy foil to the faces of the plastic substrate.
Various means have been proposed for sealing the electrolyte within its proper layer or zone in bipolar systems. Gasket members which are held by mechanical compression between the electrodes are disclosed in U.S. Pat. No. 4,637,970. In addition, use of a sealing means composed of a glue applied between the electrodes and a casing enclosing the electrodes was suggested. In U.S. Pat. No. 4,098,967 the plastic substrates forming the electrodes are molded with enlarged perimeters having mating, stepped edges which interfit to provide a seal between adjacent electrodes. In U.S. Pat. No. 4,208,473 an elaborate system of tortuous-pathed, e.g., finned, peripheral sections are provided on the electrodes to reduce parasitic current flow between adjacent cells due to electrolyte films which develop around intercell barriers.
3. Objectives
It is a principal objective of the invention to provide improvements in electrochemical systems or batteries of the type employing bipolar electrodes, whereby the system will yield an exceptionally high current capacity pulse in the 0.2 to 5 millisecond time range.
One improvement comprises a more efficient conductive substrate plate for the bipolar electrodes, wherein the improved plate has exceptionally low internal resistance that contributes to the high current capacity of the battery.
Another improvement comprises the formation of enhanced layers of pure electrochemically active material on the substrate plate to augment the action of the improved substrate plate in producing bipolar electrodes which have exceptionally high current generation capability.
A further improvement comprises a more efficient separator used to contain the electrolyte, wherein the improved separator enhances oxygen gas transmission from the positive electrode where the oxygen gas is generated to the negative electrode where the oxygen gas recombines with the electrochemically active material at that electrode.
A still further improvement comprises a novel sealing mechanism including spacer and sealing rings made of a thermoplastic material, with the rings surrounding the space or zone between electrodes which contain the electrolyte, with the sealing rings being heat fused and sealed to the peripheral portion of the thermoplastic plates to form a positive, liquid tight seal.